Integrated MEMs inertial sensing device with automatic gain control

ABSTRACT

An integrated MEMS inertial sensing device can include a MEMS inertial sensor with a drive loop configuration overlying a CMOS IC substrate. The CMOS IC substrate can include an AGC loop circuit coupled to the MEMS inertial sensor. The AGC loop acts in a way such that generated desired signal amplitude out of the drive signal maintains MEMS resonator velocity at a desired frequency and amplitude. A benefit of the AGC loop is that the charge pump of the HV driver inherently includes a ‘time constant’ for charging up of its output voltage. This incorporates the Low pass functionality in to the AGC loop without requiring additional circuitry.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to and incorporates byreference, for all purposes, the following patent application: U.S.Provisional App. 61/755,451, filed Jan. 22, 2013 and U.S. ProvisionalApp. 61/755,450, filed Jan. 22, 2013.

BACKGROUND OF THE INVENTION

The present invention is directed to MEMS(Micro-Electro-Mechanical-Systems). More specifically, embodiments ofthe invention provide methods and structure for improving integratedMEMS devices, including inertial sensors and the like. Merely by way ofexample, the MEMS device can include at least an accelerometer, agyroscope, a magnetic sensor, a pressure sensor, a microphone, ahumidity sensor, a temperature sensor, a chemical sensor, a biosensor,an inertial sensor, and others. But it will be recognized that theinvention has a much broader range of applicability.

Research and development in integrated microelectronics have continuedto produce astounding progress in CMOS and MEMS. CMOS technology hasbecome the predominant fabrication technology for integrated circuits(IC). MEMS, however, continues to rely upon conventional processtechnologies. In layman's terms, microelectronic ICs are the “brains” ofan integrated device which provides decision-making capabilities,whereas MEMS are the “eyes” and “arms” that provide the ability to senseand control the environment. Some examples of the widespread applicationof these technologies are the switches in radio frequency (RF) antennasystems, such as those in the iPhone™ device by Apple, Inc. ofCupertino, Calif., and the Blackberry™ phone by Research In MotionLimited of Waterloo, Ontario, Canada, and accelerometers insensor-equipped game devices, such as those in the Wii™ controllermanufactured by Nintendo Company Limited of Japan. Though they are notalways easily identifiable, these technologies are becoming ever moreprevalent in society every day.

Beyond consumer electronics, use of IC and MEMS has limitlessapplications through modular measurement devices such as accelerometers,gyroscopes, actuators, and sensors. In conventional vehicles,accelerometers and gyroscopes are used to deploy airbags and triggerdynamic stability control functions, respectively. MEMS gyroscopes canalso be used for image stabilization systems in video and still cameras,and automatic steering systems in airplanes and torpedoes. BiologicalMEMS (Bio-MEMS) implement biosensors and chemical sensors forLab-On-Chip applications, which integrate one or more laboratoryfunctions on a single millimeter-sized chip only. Other applicationsinclude Internet and telephone networks, security and financialapplications, and health care and medical systems. As describedpreviously, ICs and MEMS can be used to practically engage in varioustype of environmental interaction.

Although highly successful, ICs and in particular MEMS still havelimitations. Similar to IC development, MEMS development, which focuseson increasing performance, reducing size, and decreasing cost, continuesto be challenging. Additionally, applications of MEMS often requireincreasingly complex microsystems that desire greater computationalpower. Unfortunately, such applications generally do not exist. Theseand other limitations of conventional MEMS and ICs may be furtherdescribed throughout the present specification and more particularlybelow.

From the above, it is seen that techniques for improving operation ofintegrated circuit devices and MEMS are highly desired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to MEMS(Micro-Electro-Mechanical-Systems). More specifically, embodiments ofthe invention provide methods and structure for improving integratedMEMS devices, including inertial sensors and the like. Merely by way ofexample, the MEMS device can include at least an accelerometer, agyroscope, a magnetic sensor, a pressure sensor, a microphone, ahumidity sensor, a temperature sensor, a chemical sensor, a biosensor,an inertial sensor, and others. But it will be recognized that theinvention has a much broader range of applicability.

The present invention includes a drive loop configuration for anintegrated MEMS inertial sensing device. According to an embodiment, aGyroscope or inertial sensing system has a drive element that needs toresonate continuously at desired frequency and amplitude. To maintainthis oscillation of MEMS element, the drive loop invented consists of aSignal acting as driving force to a MEMS resonator, the feedback signalfrom MEMS sensor, amplifier CSA_DRV, 90 degree phase shifter, rectifier,Proportional Integral Derivative (PID) controller, comparator, chargepump providing supply voltage to the High-Voltage (HV) driver and thesignal input to the HV driver that is generated from the feedbackelement of MEMS driver.

The CSA_DRV senses the change in capacitance due to drive element andconverts it in to voltage signal. In order to provide in-phase feedbacksignal, a 90 degree phase shifter, PS0, is added in the drive loop. The90 degree phase shift can be implemented as differentiator or integratoror other known techniques.

The rectifier block rectifies the signal from phase shifter. A Low passfilter may typically be applied to this signal to provide averageenvelope of the detected feedback signal. The average amplitude is thencompared with the desired amplitude provided by the reference signalcoming from band-gap or similar on-chip or off-chip reference in theProportional, Integral, Derivative (PID) controller. The PID blockprovides multiple functionalities in the Automatic Gain Control (AGC)loop. During the normal operation, when the loop is closed, the outputof the PID block is proportional to the difference in amplitude between‘magnitude’ of the detected signal (based on envelope informationprovided by rectifier), to the reference signal Vref. The proportionalfunctionality may have some gain or may be unity. In order for the loopto filter out fast transients and act on ‘average’ information, the PIDblock incorporates an ‘Integrator’. The time constant of the integratoris kept programmable so that the AGC loop can either be made faster orslower. However, before the drive loop is closed, for example rightafter power-up of Gyro, it is desired that the AGC loop act much fasterinstead of averaging for a long time. This functionality is implementedby the ‘Differentiator’ of the PID block. The differentiator allowsshorter time to close the loop which helps in smaller ‘start-up time’for the Gyroscope.

When the MEMS drive resonator generates signal at lower amplitude thandesired, the AGC loop needs to act in a way that will increase the forceto the MEMS driver resonator which will, in turn, increase the feedbacksignal. On the other hand when the MEMS drive resonator generates signalat higher amplitude than desired, the AGC loop needs to act in a waythat will decrease the force to the MEMS driver resonator which will, inturn, decrease the feedback signal.

This invention proposes an implementation that implements this AGCmechanism in an efficient way at low power.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the present invention, reference ismade to the accompanying drawings. Understanding that these drawings arenot to be considered limitations in the scope of the invention, thepresently described embodiments and the presently understood best modeof the invention are described with additional detail through use of theaccompanying drawings in which:

FIG. 1 is a simplified diagram illustrating a cross-sectional view of anintegrated MEMS inertial sensing device according to an embodiment ofthe present invention.

FIG. 2 is a simplified block diagram illustrating an AGC loop of anintegrated MEMS inertial sensing device according to an embodiment ofthe present invention.

FIG. 3 is a simplified block diagram illustrating a charge pump of anintegrated MEMS inertial sensing device according to an embodiment ofthe present invention.

FIG. 4 is a simplified block diagram illustrating a system having anintegrated MEMS gyroscope architecture according to an embodiment of thepresent invention.

FIG. 5 is a simplified block diagram illustrating a system having anintegrated MEMS gyroscope architecture according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to MEMS(Micro-Electro-Mechanical-Systems). More specifically, embodiments ofthe invention provide methods and structures for improving integratedMEMS devices, including inertial sensors and the like. Merely by way ofexample, the MEMS device can include at least an accelerometer, agyroscope, a magnetic sensor, a pressure sensor, a microphone, ahumidity sensor, a temperature sensor, a chemical sensor, a biosensor,an inertial sensor, and others. But it will be recognized that theinvention has a much broader range of applicability.

FIG. 1 is a simplified diagram illustrating a cross-sectional view of anMEMS gyroscope device according to an embodiment of the presentinvention. The integrated MEMS gyroscope device 100 includes a substrate110 having a surface region 112, and a CMOS IC layer 120 overlyingsurface region 112 of substrate 110. CMOS IC layer 120 has a CMOSsurface region 130. In some embodiments, CMOS IC layer 120 can includeCMOS devices in substrate 110 and can including multilevel metalinterconnect structures. The example shown in FIG. 1 includes six metallayers, M1-M6. The integrated MEMS gyroscope device 100 also includes aMEMS gyroscope 140 overlying the CMOS surface region, and includes anout-of-plane sense plate 121. Integrated MEMS gyroscope device 100 alsoincludes metal shielding within a vicinity of the MEMS device configuredto reduce parasitic effects. In the example of FIG. 1, metal regions 151and 152 are shields on the sides of the plate, while 153 is the shieldbelow the plate on metal 4. Those of ordinary skill in the art willrecognize other variations, modifications, and alternatives.

The present invention includes a drive loop configuration for anintegrated MEMS inertial sensing device (i.e. gyroscope). According toan embodiment, a Gyroscope or inertial sensing system has a driveelement that needs to resonate continuously at desired frequency andamplitude. To maintain this oscillation of MEMS element, the drive loopinvented consists of a Signal acting as driving force to a MEMSresonator, the feedback signal from MEMS sensor, amplifier CSA_DRV, 90degree phase shifter, rectifier, Proportional Integral Derivative (PID)controller, comparator, charge pump providing supply voltage to theHigh-Voltage (HV) driver and the signal input to the HV driver that isgenerated from the feedback element of MEMS driver. Embodiment of thedrive loop, or Automatic Gain Control (AGC) loop circuit, is shown inFIG. 2.

The CSA_DRV senses the change in capacitance due to drive element andconverts it in to voltage signal. In order to provide in-phase feedbacksignal, a 90 degree phase shifter, PS0, is added in the drive loop. The90 degree phase shift can be implemented as differentiator or integratoror other known techniques.

The rectifier block rectifies the signal from phase shifter. A Low passfilter may typically be applied to this signal to provide averageenvelope of the detected feedback signal. The average amplitude is thencompared with the desired amplitude provided by the reference signalcoming from band-gap or similar on-chip or off-chip reference in theProportional, Integral, Derivative (PID) controller. The PID blockprovides multiple functionalities in the Automatic Gain Control (AGC)loop. During the normal operation, when the loop is closed, the outputof the PID block is proportional to the difference in amplitude between‘magnitude’ of the detected signal (based on envelope informationprovided by rectifier), to the reference signal Vref. The proportionalfunctionality may have some gain or may be unity. In order for the loopto filter out fast transients and act on ‘average’ information, the PIDblock incorporates an ‘Integrator’. The time constant of the integratoris kept programmable so that the AGC loop can either be made faster orslower. However, before the drive loop is closed, for example rightafter power-up of Gyro, it is desired that the AGC loop act much fasterinstead of averaging for a long time. This functionality is implementedby the ‘Differentiator’ of the PID block. The differentiator allowsshorter time to close the loop which helps in smaller ‘start-up time’for the Gyroscope.

When the MEMS drive resonator generates signal at lower amplitude thandesired, the AGC loop needs to act in a way that will increase the forceto the MEMS driver resonator which will, in turn, increase the feedbacksignal. On the other hand when the MEMS drive resonator generates signalat higher amplitude than desired, the AGC loop needs to act in a waythat will decrease the force to the MEMS driver resonator which will, inturn, decrease the feedback signal.

This invention proposes an implementation that implements this AGCmechanism in an efficient way at low power.

Output of the PID block is proportional to the difference between thesignal received from the drive feedback (CSA_DRV) and desired reference.In this invention, a Pulse Width Modulated (PWM) signal is derived basedon output of PID block.

A triangle wave generator creates triangular pulses. For example, thiswaveform may be implemented by linearly charging & dischargingcapacitor. The triangular waveform is then compared with output of thePID in the ‘comparator’ block.

If the PID block output is at desired level (Vref), then the comparatoroutputs PWM pulses that maintain current value out of the charge pump.If the PID block output is lower than desired, the duty cycle of the PWMpulses is effectively increased. If the PID block output is higher thandesired, the duty cycle of the PWM pulses is effectively reduced.

Output of charge pump is proportional to the PWM duty cycle. Forexample, charge pump architecture is designed in order to increase thecharge pump output amplitude when duty cycle of PWM is higher than 50%,reduce it when duty cycle is less than 50% and maintain the output atthe current value when the duty cycle is 50%.

FIG. 3. shows one embodiment that allows implementation of thefunctionality described above for PID and charge pump CP1 combinedtogether. For this implementation, both the PWM pulse and its invertedpulse, referred as “inv-PWM” are used. The When the PWM output is HIGHvalue, then the current source C1 sources current IA and adds to thecharge on capacitor C_chgpmp. The capacitor C_chgpmp may be on-chipcapacitor, off-chip capacitor or partially on-chip and partiallyoff-chip capacitor. Adding more charge increases value of V_CP voltage.In the time duration when the PWM pulse is LOW, inv_PWM is HIGH. In thistime duration, current source C2 drains current of amplitude IA from thecapacitor C_chgpmp. This reduces voltage V_CP.

When the duty cycle of PWM pulse is large than 50%, C1 adds more chargeson C_chgpmp than the charge removed by current source C2. This willincrease value of V_CP. On the other hand, When the duty cycle of PWMpulse is smaller than 50%, C1 adds less charges on C_chgpmp than thecharge removed by current source C2. This will reduce V_CP. When theduty cycle is exactly 50%, the added and deleted charges are equal andthe voltage V_CP will remain at its current value.

The “Integration” functionality of PID is automatically implemented bythe time constant proportional to C_chgpmp and the value of IA. TheDifferentiator functionality is implemented by changing IA in thestart-up duration so that the time constant is reduced and the V_CPvoltage changes fast thereby reducing ‘start-up time’.

Another innovation is that C_chgpmp may be driven by a charge pump CP2(shown in FIG. 5) in addition to the capacitor C_chgpmp and the currentsources. CP2 will pump up voltage from external low voltage (e.g. 1.8V)to desired high voltage (e.g. 10V). CP2 helps in maintaining a desiredvoltage on V_CP. The circuits constituting current sources C1, C2contribute in either incrementing, decrementing or maintaining thenominal value from the charge pump.

Another inventive aspect is that the AGC is implemented as combinationof Charge pump acting as power supply to the HV driver. The HV drivercan be either analog HV amplifier or simple inverter. The supply voltageof HV driver is provided from charge pump CP1 and provides one means ofcontrolling the output amplitude from HV driver.

The maximum value or amplitude of the HV driver is then effectivelycontrolled by the charge pump. In one embodiment, the input to the HVdiver may be a pulse. The output of the HV driver can pulse whoseamplitude is controlled by the charge pump. Another embodiment is thatthe HV driver can simply be a HV inverter. Charge pump powers the HVdriver block. If charge pump output is higher, the HV driver outputsproportionally higher amplitude pulses which will inject more Force,proportional to product of dc and ac voltage output from HV driver, into MEMS driver-resonator. The displacement generated by the resonator isproportional to the input force and the Q of the resonator. E.g. largerthe Q, larger is the displacement. Also, for a given Q, larger theforce, larger is the displacement of MEMS drive element. Largerdisplacement of MEMS element generates larger signal (for example ascapacitance change). Thus, the AGC loop acts in a way that generateddesired signal amplitude out of the drive signal and equivalently,maintains MEMS resonator velocity as desired frequency and amplitude.

The PID block also provides Differential signal which is necessary forkick-start of the AGC loop in order to pump up the charge pump outputfaster especially during power on. In the normal mode, an integratorintegrates the output of PID so that noise pulses do not cause undesiredchanges in the AGC path.

One of the advantages of proposed AGC loop is that the charge pump,inherently includes a ‘time constant’ for charging up of its outputvoltage. This incorporates the Low pass functionality in to the AGC loopwithout requiring additional circuitry.

FIG. 4 is a simplified block diagram illustrating a system having anintegrated MEMS gyroscope architecture according to an embodiment of thepresent invention. Some included components are the Charge SenseAmplifiers (CSA), Programmable Gain Amplifier (PGA), Low Pass Filter(LPF), I2C. The CSAs is used for processing signals from the drive pathas well as sense path of a Gyroscope or other MEMS inertial sensingdevice. The I2C is a serial bus communication to digital registers onthe chip.

The MEMS block shown in FIG. 4 is single or multi-axis MEMS gyroscopeelement. The sensing element is shown as capacitive but other sensingelements are also possible and the first amplifier interfacing with theMEMS element is designed appropriately.

The Gyroscope has a drive element that needs to resonate continuously atdesired frequency and amplitude. To maintain this oscillation of MEMSelement, the drive loop consisting of CSA_DRV, phase shifter (PS1),comparator, HV driver provides gain of signal at desired frequency ofoscillation. The CSA_DRV senses the change in capacitance due to driveelement and converts it in to voltage signal.

In order to provide in-phase feedback signal, a 90 degree phase shifter,PS0, is added in the drive loop. The 90 degree phase shift can beimplemented as differentiator or integrator or other known techniques.

The Rectifier, comparator, Proportional-Integral-Derivative (PID)controller, High Voltage (HV) driver form an Automatic Gain Control(AGC) loop. When the MEMS drive resonator generates signal at loweramplitude than desired, the amplitude of the rectified signal from theCSA, used for processing signal from drive path as well as sense path ofGyroscope, is smaller compared to the reference signal provided to thePID. The PID block generates output in Proportion to the difference ofthe input signals. The output of the PID block drives the charge pump.Output of PID block will be proportional to the difference in referencevoltage input to the PID and the rectified signal amplitude. If outputof PID is higher, then charge will provide larger voltage output. Thecharge pump powers the HV driver block. If charge pump output is higher,the HV driver outputs proportionally higher amplitude pulses which willinject more Force, proportional to product of dc and ac voltage outputfrom HV driver, in to MEMS driver-resonator. The displacement generatedby the resonator is proportional to the input force and the Q of theresonator. E.g. larger the Q, larger is the displacement. Also, for agiven Q, larger the force, larger is the displacement of MEMS driveelement. Larger displacement of MEMS element generates larger signal(for example as capacitance change). Thus, the AGC loop acts in a waythat generated desired signal amplitude out of the drive signal andequivalently, maintains MEMS resonator velocity as desired frequency andamplitude.

The PID block also provide a differential signal, which is necessary forkick-start of the AGC loop in order to pump up the charge pump outputfaster especially during power on. In the normal mode, an integratorintegrates the output of PID so that noise pulses do not cause undesiredchanges in the AGC path and makes the steady state error to be zero.

The rectifier, comparator, Proportional-Integral-Derivative (PID)controller, High Voltage (HV) driver, MEMS resonator, CSA_DRV and the 90degree phase shifter, PS0, form an Automatic Gain Control (AGC) loop.When the MEMS drive resonator generates signal at lower amplitude thandesired, the amplitude of the rectified signal from the CSA is smallercompared to the reference signal provided to the PID. The PID blockgenerates output in proportion to the difference of the input signals.The output of the PID block controls the gain of the HV driver, whichdecides the amplitude of the ac voltage, Vac, driving the MEMSresonator.

By driving the MEMS drive capacitors with an AC voltage that is of lowimpedance, dependence of the AGC loop performance on MEMS leakage can besubstantially eliminated. The displacement generated by the resonator isproportional to the input force and the Q of the resonator. E.g. largerthe Q, larger is the displacement. Also, for a given Q, the larger theforce, the larger is the displacement of MEMS drive element. Largerdisplacement of the MEMS element generates a larger signal (for exampleas capacitance change). Thus, the AGC loop acts in a way that generatesa desired signal amplitude out of the drive signal and equivalently,maintains MEMS resonator velocity as desired frequency and amplitude.

The PID block also provide a differential signal, which is necessary forkick-start of the AGC loop in order to pump up the charge pump outputfaster especially during power on. In the normal mode, an integratorintegrates the output of PID so that noise pulses do not cause undesiredchanges in the AGC path and makes the steady state error to be zero.

In a specific embodiment, the sensing mechanism of the gyroscope isbased on a Coriolis force, which is proportional to the vector productof angular rate of the gyroscope and the velocity of the MEMS driverresonator. The Coriolis force generates a displacement of the MEMS senseelement in a direction that is orthogonal to the drive velocity and theexternal angular rate. The displacement signal is sensed via a senseCharge Sense Amplifier (CSA). The signal at the output of the sense CSAwill have a carrier signal at the frequency of the resonance of thedrive resonator, which will be amplitude modulated by a signalproportional to the angular rate of motion.

The drive signal also gets injected in the sense path and is 90 degreesout of phase compared to the Coriolis displacement, and hence is termed“Quadrature coupling”. A programmable Quadrature cancellation DAC is anarray of programmable capacitors that allow a desired portion of thequadrature signal to be cancelled from the input signal. In addition, toaccurately cancel the quadrature, which may have a different phase than90 deg, a phase shifter PS1 is used in the present architectureembodiment.

The CSA-sense is a low noise amplifier with capacitive feedback. Inorder to maintain DC biasing at the amplifier input, a very highimpedance feedback at low frequency is required. In various embodimentsof the present invention, this configuration is realized by using MOStransistors operating in a sub-threshold region that can createimpedances in the order of Giga-ohms. The feedback to maintain inputcommon voltage is only desired at DC. In order to ensure little impactof the high impedance common mode feedback and to minimize noise impactat high frequency, a very low cut-off frequency low pass filter is addedin the feedback path.

The Programmable Gain Amplifier 1 (PGA1) amplifies the signal from CSAto a desired level. The rate signal needs to be demodulated from thesignal at the output of sense CSA. The mixer in the signal path achievesthe demodulation by mixing the carrier signal coming out from the driveCSA with the composite signal coming from the sense-CSA amplified by thePGA.

In a specific embodiment, the mixer is implemented as a transmissiongate. One input of the transmission gate is the pulse coming from thecomparator after going through the programmable phase shifter PS2. PhaseShifter PS2 shifts pulses by a programmable amount from −180 deg to +180deg thereby adjusting for both phase lead and lag between signal indrive loop verses signals in sense path.

The Programmable Gain Amplifier PGA2 amplifies the demodulated ratesignal. PGA2 also includes a Low Pass Filtering function. An embodimentof the present invention includes a LPF by simply adding a capacitor inthe feedback path of the PGA2. Since the carrier component is suppressedwith the LPF in the PGA2, the dynamic range can be effectively used fordesired rate signal amplification before converting to a digital domain.

A small phase shift with respect to 90 degrees may exist in the CSAdrive, which will generate DC or low frequency components afterdemodulation, thereby consuming dynamic range after the mixer. Accordingto a specific embodiment, a programmable phase shifter, PS2, can beconfigured within the architecture to effectively cancel this component.

According to another specific embodiment, a loop including or consistingof a digital low pass filter and DAC2 can be provided within thearchitecture. This loop cancels a small offset or low frequencycomponent that may exist in the signal path due to offsets of analogblocks or DC or low frequency components produced by the mixer that isnot in the range of rate signal frequency.

A high resolution (e.g. 16 bit) A/D converter (ADC) converts thedemodulated rate signal. The A/D converter has inputs for multiplechannels in order to multiplex the digital signal path for all of thechannels. One of the inputs of the A/D converter is from the on-chiptemperature sensor. The Temp sensor output can be effectively used tocompensate for the effect of the resonator variation with temperatureeither in the analog or digital domain. In an embodiment, thetemperature sensor output can be read and used to program the phaseshifter PS2 to compensate for changes in phase occurring due to changesin temperature. Temperature compensation can also be applied in thedigital path with certain programmability. Also, multiple axes (e.g.three axis for a 3 degrees of freedom (3DOF) Gyro) of the Gyro signalare multiplexed at the ADC.

The digital path can have signal processing such as programmable LowPass Filters to cancel noise outside of a band of interest. The digitalsignal path also has a programmable High Pass Filter (HPF) tosubstantially eliminate DC components, offset, or very low frequencyartifacts that are not within the expected rate signal band.

The system architecture of FIG. 4 also shows a test mode that allowsmeasurement of quadrature signal using blocks QD, comparator andmultiplexer. In this embodiment, a voltage corresponding to the drivedisplacement is used to demodulate the signal form the MEMS sensecapacitors. This mode is multiplexed with the ‘normal’ mode in which thevoltage corresponding to the drive velocity is used to demodulate thesignal from the MEMS sense capacitors. The quadrature mode provides amethod to quantify the residual ‘feed-through’ or ‘quadrature’ signalfrom the MEMS drive capacitors to the sense capacitors, and to observehow it varies with ambient parameters such as temperature, humidity,etc.

In a specific embodiment, a digital delay, using block DEL1, isintroduced in the frequency control loop of the drive servo. Varying thedelay causes the loop to lock into different frequencies. For instance,the delay can be varied to make the loop lock into the 3-dB frequenciesof the MEMS drive resonator and measure the 3 dB bandwidth and qualityfactor.

FIG. 5 is a simplified block diagram illustrating a system having anintegrated MEMS gyroscope architecture according to an embodiment of thepresent invention. Similar to FIG. 4, the MEMS block shown in FIG. 5 issingle or multi-axis MEMS gyroscope element. The sensing element isshown as capacitive but other sensing elements are also possible and thefirst amplifier interfacing with the MEMS element is designedappropriately. Several of the components discussed for FIG. 4 are alsopresent in the embodiment depicted by FIG. 5. Thus, further informationregarding of these components can be found in the previous descriptions.

One of the benefits of proposed AGC loop is that the charge pump,inherently includes a ‘time constant’ for charging up of its outputvoltage. This incorporates the Low pass functionality in to the AGC loopwithout requiring additional circuitry.

The HV driver can be either analog HV amplifier or simple inverter. Thesupply voltage of HV driver is provided from charge pump CP1 andprovides one means of controlling the output amplitude from HV driver.

An additional charge pump, CP2, is designed to allow external powersupply voltage that can be lower compared to on-chip voltages. Forexample, the external power can be 1.8V and internal voltages can be3.3V and much higher voltages at charge pump for HV driver.

The multiple charge pump architecture allows more efficient usage ofpower. For example, the boosting of external supply voltage 1.8V mayhave to be boosted to 32V. This can be done as boost from 1.8V to 3.3Vand from 3.3V to 32V. This feature will allow usage of device at highervoltage to bypass one of the charge pumps CP2.

The layout of the Gyroscope MEMS and CMOS is very critical to achieveoptimal performance. All the out of plane sense signal plates areshielded with metal shield on sides (on same metal layer) as well as onlayers below the sense plates. In a specific embodiment, a shield may beplaced by skipping one or more metal layers to minimize parasiticcapacitance. For example, if sense plate is on metal 6, the shield maybe on metal 4 instead of metal 5 in order to provide more isolation &reduce parasitics. An example is shown in FIG. 1, which shows that CMOSIC layer 120 can include CMOS devices (not shown) in substrate 110 andcan including multilevel metal interconnect structures, e.g., six metallayers, M1-M6. The integrated MEMS gyroscope device 100 also includes aMEMS gyroscope 140 overlying the CMOS surface region, and includes anout-of-plane sense plate 121. Metal regions 151 and 152 are shields onthe sides of the plate in the metal 6 layer, while 153 is the shieldbelow the plate on the metal 4 layer.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. An integrated MEMS inertial sensing device, thedevice comprising: a substrate member having a surface region; a CMOS IClayer overlying the surface region, the CMOS IC layer having a CMOSsurface region, the CMOS IC layer having an Automatic Gain Control (AGC)loop circuit, the AGC loop circuit including a rectifier, aproportional-integral-derivative (PID) controller, an input of the PIDcontroller being electrically connected to an output of the rectifier, acomparator having an input electrically connected to an output of thePID controller, a charge pump having an input electrically connected toan output of the comparator, and a high-voltage (HV) driver having aninput electrically connected to an output of the charge pump; and a MEMSinertial sensor overlying the CMOS surface region, the MEMS inertialsensor electrically coupled to the AGC loop circuit.
 2. The integratedMEMS inertial sensing device of claim 1 wherein the PID controllerincludes an integrator and a differentiator, wherein the integrator usesa programmable time constant to determine a variable operational speedof the AGC loop circuit, and wherein the differentiator is configured tochange a current for charging and discharging a capacitor in the chargepump in the start-up duration so the time constant is reduced and thecapacitor voltage changes faster to reduce a start-up time of the AGCloop circuit.
 3. The integrated MEMS inertial sensing device of claim 1wherein the PID controller is configured to output a differential PWMsignal having a PWM signal and an inverted PWM signal, and wherein theoutput of the charge pump is configured to be proportional to a dutycycle of the PWM signal.
 4. The integrated MEMS inertial sensing deviceof claim 3 wherein the charge pump is a first charge pump, and the AGCloop further comprises a second charge pump coupled to a charge pumpnode, wherein a first current source is coupled to the charge pump node,the PWM signal, and a supply voltage, and wherein a second currentsource is coupled to the charge pump node, the inverted PWM signal, andground.
 5. The integrated MEMS inertial sensing device of claim 4further comprising a charge pump capacitor coupled to the charge pumpnode, the second charge pump, the first and second current sources, andthe charge pump capacitor being configured to maintain a desired voltageon the charge pump node.
 6. The integrated MEMS inertial sensing deviceof claim 1 further comprising a triangle wave generator coupled to thecomparator and configured to generate triangular pulses, wherein theoutput of the PID is compared to the triangular pulses by thecomparator.
 7. The integrated MEMS inertial sensing device of claim 1wherein the charge pump is configured as a power supply to the HVdriver.
 8. An integrated MEMS inertial sensing device, the devicecomprising: a substrate member having a surface region; a CMOS IC layeroverlying the surface region, the CMOS IC layer having a CMOS surfaceregion, the CMOS IC layer having an Automatic Gain Control (AGC) loopcircuit, the AGC loop circuit including a rectifier, aproportional-integral-derivative (PID) controller, an input of the PIDcontroller being electrically connected to an output of the rectifier, acomparator having an input electrically connected to an output of thePID controller, a charge pump having an input electrically connected toan output of the comparator, and a high-voltage (HV) driver having aninput electrically connected to an output of the charge pump; and a MEMSinertial sensor overlying the CMOS surface region, the MEMS inertialsensor electrically coupled to the AGC loop circuit through a driverresonator in the AGC loop; wherein the PID controller includes anintegrator and a differentiator, wherein the integrator uses aprogrammable time constant to determine a variable operational speed ofthe AGC loop circuit, and wherein the differentiator is configured tochange a current for charging and discharging a capacitor in the chargepump in the start-up duration so the time constant is reduced and thecapacitor voltage changes faster to reduce a start-up time of the AGCloop circuit.
 9. The integrated MEMS inertial sensing device of claim 8further comprising a mixer coupled to the MEMS inertial sensor, whereinthe mixer is configured as a transmission gate.
 10. The integrated MEMSinertial sensing device of claim 9 further comprising a circuit loopincluding a digital low-pass-filter (LPF) coupled to a digital/analogconverter (DAC), the circuit loop being coupled to the mixer.
 11. Theintegrated MEMS inertial sensing device of claim 8 further comprising aprogrammable phase-shifter (PS) coupled to the comparator.
 12. Theintegrated MEMS inertial sensing device of claim 8 wherein thecomparator is a first comparator, and further comprising a quadraturemode circuit including a second comparator and a multiplexer, whereinthe quadrature mode circuit is configured to monitor a quadrature signalfrom the MEMS inertial sensor.
 13. The integrated MEMS inertial sensingdevice of claim 8 further comprising a digital delay module coupled tothe comparator and the HV driver, the digital delay module beingconfigured to lock into a desired frequency.
 14. The integrated MEMSinertial sensing device of claim 8 further comprising metal shieldingwithin a vicinity of the MEMS inertial sensor, the metal shielding beingconfigured to reduce parasitic effects.
 15. An integrated MEMS inertialsensing device, the device comprising: a substrate member having asurface region; a CMOS IC layer overlying the surface region, the CMOSIC layer having a CMOS surface region, the CMOS IC layer having anAutomatic Gain Control (AGC) loop circuit, the AGC loop circuitincluding a rectifier, a proportional-integral-derivative (PID)controller, an input of the PID controller being electrically connectedto an output of the rectifier, a comparator having an input electricallyconnected to an output of the PID controller, a charge pump having aninput electrically connected to an output of the comparator, and ahigh-voltage (HV) driver having an input electrically connected to anoutput of the charge pump; and a MEMS inertial sensor overlying the CMOSsurface region, the MEMS inertial sensor electrically coupled to the AGCloop circuit; wherein the PID controller includes an integrator and adifferentiator, wherein the integrator uses a programmable time constantto determine a variable operational speed of the AGC loop circuit, andwherein the differentiator is configured to change a current forcharging and discharging a capacitor in the charge pump in the start-upduration so the time constant is reduced and the capacitor voltagechanges faster to reduce a start-up time of the AGC loop circuit; andwherein the PID controller is configured to output a differential PWMsignal having a PWM signal and an inverted PWM signal, and wherein theoutput of the charge pump is configured to be proportional to a dutycycle of the PWM signal.
 16. The integrated MEMS inertial sensing deviceof claim 15 wherein the charge pump is a first charge pump, and furthercomprising a second charge pump coupled to a charge pump node, wherein afirst current source is coupled to the charge pump node, the PWM signal,and a supply voltage, and wherein a second current source is coupled tothe charge pump node, the inverted PWM signal, and ground.
 17. Theintegrated MEMS inertial sensing device of claim 16 further comprising acharge pump capacitor coupled to the charge pump node, the second chargepump, the first and second current sources, and the charge pumpcapacitor being configured maintain a desired voltage on the charge pumpnode.
 18. The integrated MEMS inertial sensing device of claim 15further comprising a triangle wave generator coupled to the comparatorand configured to generate triangular pulses, wherein the output of thePID is compared to the triangular pulses by the comparator.
 19. Theintegrated MEMS inertial sensing device of claim 15 wherein the chargepump is configured as a power supply to the HV driver.